Dimmer circuits are used to control the power provided to a load such as a light or electric motor from a power source such as mains. Such circuits often use a technique referred to as phase controlled dimming. This allows power provided to the load to be controlled by varying the amount of time that a switch connecting the load to the power source is conducting during a given cycle.
For example, if voltage provided by the power source can be represented by a sine wave, then maximum power is provided to the load if the switch connecting the load to the power source is on at all times. In this way the, the total energy of the power source is transferred to the load. If the switch is turned off for a portion of each cycle (both positive and negative), then a proportional amount of the sine wave is effectively isolated from the load, thus reducing the average energy provided to the load. For example, if the switch is turned on and off half way through each cycle, then only half of the power will be transferred to the load. Because these types of circuits are often used with resistive loads and not inductive loads, the effect of repeatedly switching on and off power will not be noticeable as the resistive load has an inherent inertia to it. The overall effect will be, for example in the case of a light, a smooth dimming action resulting in the control of the luminosity of the light. This technique will be well understood by the person skilled in the art.
In leading edge dimmer topologies for controlling inductive loads, it is generally necessary to allow the prevailing half-cycle load current to fall to near-zero levels, before returning the switch to the off-state, in order to avoid excessive inductive voltage spiking levels, which can cause damage to the electrical components of the dimmer circuit and the load. Turning off the switch while there is any appreciable level of current causes a sudden rise in the voltage appearing across the load. As described by the well known relationshipV=L*dI/dt Where V is the voltage appearing across the inductive load;                L is the magnitude of the inductance of the load; and        dI/dt is the rate of change of the current I through the load over time tas will be understood by the person skilled in the art.        
As can be seen, the greater the rate of change in current I through the load, the greater the voltage spike occurring. It follows then that the greater the current at the time of turning off the switch, which causes the current to fall to zero in a very short space of time, the greater the rate of change of current and therefore the greater the voltage spike induced. Accordingly then, it is desirable to turn off the switch when the magnitude of current is nearly zero.
Several techniques are commonly used to achieve this.
The first is the use of a series current sense resistor with a precision operational amplifier arrangement to determine when the load current falls to sufficiently low levels such that the switch can be turned off with minimal resulting voltage spiking levels. However, since the current sense resistor adds to the total resistance of the dimmer, it results in more power being drained by the dimmer. Accordingly, this approach has some draw backs.
In a second technique, use is made of the MOSFET on-state resistance. In many dimmer circuit arrangements, the switch is constituted by a MOSFET device (Metal-Oxide Semiconductor Field-Effect Transistor). This MOSFET has an internal resistance when in the on state. In a similar manner to the above, a precision op-amp arrangement can be employed to determine the point in time where the MOSFET on-state conduction voltage polarity reverses. This approach however, suffers from the draw back that the timing accuracy of zero crossing detection diminishes for smaller loads.
A third approach is to use non-zero current MOSFET turn-off anyway. For loads that are only marginally inductive (for example, typical iron core based LV lighting) it is possible to turn off the MOSFETs at the point in time corresponding to mains voltage zero-crossing. The stored energy in the transformer leakage inductance can then be allowed to transfer to a suitably sized capacitor in shunt with the dimmer terminals. A decaying ringing dimmer voltage waveform will result, where the initial stored energy is dissipated, over a number of ringing cycles, in the series resistance comprising the transformer loss elements and secondary lamp load. Since the instantaneous mains voltage is relatively low around the zero crossing region, moderate peak ringing voltages can be tolerated. Of course, this approach is only useful for loads that have only a small inductive component, and cannot be used for greater or purely inductive loads.
It is therefore an object of the present invention to provide an alternative method and circuit for reducing the occurrence of voltage spiking when controlling an inductive load or a load having an inductive component.